Conventional heat transfer fluids such as water, mineral oil, and ethylene glycol play an important role in many industries including power generation, chemical production, air conditioning, transportation, and microelectronics. However, their inherently low thermal conductivities have hampered the development of energy-efficient heat transfer fluids that are required in a plethora of heat transfer applications. It has been demonstrated recently that the heat transfer properties of these conventional fluids can be significantly enhanced by dispersing nanometer-sized solid particle and fibers (i.e. nanoparticles) in fluids (Eastman, et al., Appl. Phys. Lett. 2001, 78(6), 718; Choi, et al., Appl. Phys. Lett. 2001, 79(14), 2252). This new type of heat transfer suspensions is known as nanofluids. Carbon nanotube-containing nanofluids provide several advantages over the conventional fluids, including thermal conductivities far above those of traditional solid/liquid suspensions, a nonlinear relationship between thermal conductivity and concentration, strongly temperature-dependent thermal conductivity, and a significant increase in critical heat flux. Each of these features is highly desirable for thermal systems and together makes nanofluids strong candidates for the next generation of heat transfer fluids.
The observed substantial increases in the thermal conductivities of nanofluids can have broad industrial applications and can also potentially generate numerous economical and environmental benefits. Enhancement in the heat transfer ability could translate into high energy efficiency, better performance, and low operating costs. The need for maintenance and repair can also be minimized by developing a nanofluid with a better wear and load-carrying capacity. Consequently, classical heat dissipating systems widely used today can become smaller and lighter, thus resulting in better fuel efficiency, less emission, and a cleaner environment.
Nanoparticles of various materials have been used to make heat transfer nanofluids, including copper, aluminum, copper oxide, alumina, titania, and carbon nanotubes (Keblinski, et al, Material today, 2005, 36). Of these nanoparticles, carbon nanotubes show greatest promise due to their excellent chemical stability and extraordinary thermal conductivity. Carbon nanotubes are macromolecules of the shape of a long thin cylinder and thus with a high aspect ratio. There are two main types of carbon nanotubes: single-walled nanotubes (“SWNT”) and multi-walled nanotubes (“MWNT”). The structure of a single-walled carbon nanotube can be described as a single graphene sheet rolled into a seamless cylinder whose ends either open or capped by either half fullerenes or more complex structures including pentagons. Multi-walled carbon nanotubes comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk with a typical distance of approximately 0.34 nm between layers.
Carbon nanotubes are the most thermal conductive material known today. Basic research over the past decade has shown that carbon nanotubes could have a thermal conductivity an order of magnitude higher than copper (3,000 W/m·K for multi-walled carbon nanotubes and 6,000 W/m·K for single-walled carbon nanotubes). Therefore, the thermal conductivities of nanofluids containing such solid particles would be expected to be significantly enhanced when compared with conventional fluids along. Experimental results have demonstrated that carbon nanotubes yield by far the highest thermal conductivity enhancement ever achieved in a fluid: a 150% increase in conductivity of oil at about 1% by volume of multi-walled carbon nanotubes (Choi, et al., App. Phys. Lett., 2001, 79(14), 2252).
Several additional studies of carbon nanotube suspensions in various heat transfer fluids have since been reported. However, only moderate enhancements in thermal conductivity have been observed. Xie et al. measured a carbon nanotube suspension in an aqueous solution of organic liquids and found only 10-20% increases in thermal conductivity at 1% by volume of carbon nanotubes (Xie, et al., J. Appl. Phys., 2003, 94(8):4967). Similarly, Wen and Ding found an about 25% enhancement in the conductivity at about 0.8% by volume of carbon nanotubes in water (Wen and Ding, J. Thermophys. Heat Trans., 2004, 18:481). Even at these levels, carbon nanotubes still hold great promises of developing the next generation of efficient thermal transfer fluids.
Despite those extraordinary promising thermal properties exhibited by carbon nanotube suspensions, it remains to be a serious technical challenge to effectively and efficiently disperse carbon nanotubes into aqueous or organic mediums to produce a nanoparticle suspension with a sustainable stability and consistent thermal properties. Due to hydrophobic natures of graphitic structure, carbon nanotubes are not soluble in any known solvent. They also have a very high tendency to form aggregates and extended structures of linked nanoparticles, thus leading to phase separation, poor dispersion within a matrix, and poor adhesion to the host. However, stability of the nanoparticle suspension is especially essential for practical industrial applications. Otherwise, the thermal properties of a nanofluid, such as thermal conductivity, will constantly change as the solid nanoparticles gradually separate from the fluid. Unfortunately, these early studies on carbon nanotubes-containing nanofluids have primarily focused on the enhancement of thermal conductivity and very little experimental data is available regarding the stability of those nanoparticle suspensions.
Accordingly, there is a great need for the development of an effective formulation which can be used to efficiently disperse different forms of carbon nanotubes into a desired heat transfer fluid and produce a nanofluid with a sustainable stability and consistent thermal properties. Hence, the present invention provides a nanofluid composition, which comprises a conventional heat transfer fluid and carbon nanoparticles. The present invention also relates to methods for preparing a hydrophilic nanofluid, nanolubricant and nanogrease with enhanced thermal conductivities. Furthermore, the present invention provides a homogenous nanofluid which contains soluble nanoparticles.